Approaching Earth makes asteroids go pale

We already know that asteroid impacts can cause cataclysms on Earth – now it seems that Earth can hit back. Close encounters with our planet appear to shake up the structure of some asteroids.

The idea explains an old puzzle. Most asteroids are stained dark red by the relentless bombardment of space particles. But some asteroids that stray into the inner solar system are paler, like the interior of meteorites.

They calculated the orbital behaviour of 95 asteroids that travel into the inner solar system. Over the past half a million years, some orbits remained aloof from Earth's, and others drifted perilously close to it.

Of the 20 pale asteroids in the sample, every single one had the possibility of a close encounter with Earth. "We've nailed it," says Binzel.

Danger zone

The result implies that Earth's gravity stretches and disturbs asteroids that come too close, perhaps causing landslides that expose underlying rock. To fit the observed number of pale asteroids, the team calculates that this must happen to any asteroids that come within about 100,000 kilometres of our planet.

Their theory will be tested in 2029, when a dark asteroid called Apophis is due to come within about 40,000 km of Earth.

Binzel expects it to be resurfaced and change colour. "It should be enough to rearrange all the rubble on the surface," he told New Scientist. "It would be fascinating to have seismic probes implanted on Apophis, then fly alongside it and watch and listen as it creaks and groans its way past the Earth."

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The distance at which near-Earth asteroids approach our planet may explain their colour (Illustration: ESO)